Joshua Kenneth Robertson Tabh , Andreas Nord

Ecogeographical rules, describing common trends in animal form across space and time, have provided key insights into the primary factors driving species diversity on our planet. Among the most well-known ecogeographical rules are Bergmann’s Rule and Allen’s rule, with each correlating ambient temperature to the size and shape of endotherms within a species. These two rules have recently attracted renewed research attention, largely with the goal of understanding how they emerge (e.g. via natural selection or phenotypic plasticity), and thus, whether they may emerge quickly enough to aid adaptations to a warming world. Yet despite this attention, the precise proximate and ultimate drivers of Bergmann’s and Allen’s rules remain unresolved. In this conceptual paper, we articulate novel and classic hypotheses for understanding whether and how plastic responses to developmental temperatures might contributed to each rule. Next, we compare over a century of empirical literature surrounding Bergmann’s and Allen’s Rules against our hypotheses to uncover likely avenues by which developmental plasticity might drive temperature-phenotype correlations. Across birds and mammals, studies strongly support developmental plasticity as a driver of Bergmann’s and Allen’s rules, particularly with regards to Allen’s rule and responses to heat. However, these plastic contributions toward each appear largely non-linear and dependent upon: (1) efficiency of energy use at given ambient temperatures (Bergmann’s rule), and (2) thermal advantages at given ambient temperatures (Allen’s Rule). These findings suggest that, among endotherms, rapid changes in body shape and size will continue to occur, but generalizing the direction of responses across populations is likely naive.